You're going to be seeing a whole lot of emails in your inbox over the next couple of weeks, asking for money as the year draws to a close.

Those emails will take many different forms:

Urgent pleas to help boost totals before campaigns have to make their fundraising results public. "Dropping out," read a recent note from Texas Sen. Ted Cruz's campaign. The explanation for the alarming subject line came from an email Cruz supposedly wrote from his iPhone: "Jindal dropped out of the race b/c he was short on funds."

Outraged responses to news of the day. "Media says I'm to blame," wrote Kentucky Sen. Rand Paul in the wake of recent terrorist attacks.

And a whole lot of candidates will want to hang out with you. From former Secretary of State Hillary Clinton: "dinner?" And from former Florida Gov. Jeb Bush, "Friend, let's meet." Sometimes you'll even get an invite to win dinner with a real-life nonpolitical celebrity. "I'd love to meet you and your family," Drew Barrymore wrote to Clinton supporters earlier this month.

Testing the message

Here's the thing: as odd as some of these messages appear to be, campaigns know they work. Most political operations conduct meticulous tests to see which appeals work best, before they blast notes out to their bases.

It's an approach perfected by President Obama's 2012 re-election campaign, which hauled in about $500 million based on email fundraising.

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"Nearly every email we planned to send in 2012, we would set aside a certain percentage of subscribers, and test out anywhere from 12 to 18 different variations of that message," explained the campaign's email director, Toby Fallsgraff. "Sometimes they were really subtle changes, and sometimes they were completely different angles on that message."

Obama staffers would scour the test group's results, looking for data about what worked and what didn't. "How many people opened it? How many people unsubscribed?" said Fallsgraff. "But most importantly, I think it's how many people responded by taking the action we were asking them to take."

50 different subject lines

The Clinton campaign is clearly following the Obama 2012 lead. In November alone, it used 50 different subject lines to ask supporters to give money and enter contests to win dinner with either Bill or Hillary.

That comes from data provided to NPR by Return Path, an email marketing firm that senior research director Tom Sather characterizes as "the Nielsen of email," comparing it to the company that surveys who's watching what TV shows when.

Sather says Return Path's panel of subscribers, who pass along anonymous information from their inboxes to the company, don't get every single email that campaigns send out, but he's confident "about 95 percent of all messages being sent" end up in the company's databases.

Subjects ranged from "Bill wants to meet you" to "dinner!" to "dinner?"

The approach worked. According to Return Path, of the five top most-read emails the Clinton campaign sent out in November, four of them had the subject line "dinner!"

When emails get too human

That doesn't surprise Fallsgraff, who said "emails that are effective — that we've seen be effective — are human. They feel like a human interaction."

But when campaign staffers are strategically typing out messages that are supposedly from a high-profile candidate, there's such a thing as too human.

Rand Paul supporter Mark English of White Plains, N.Y., was irked when he received an October email with the subject line "Fw: Please reach out to Mark."

"Mark, are you ok? Rand asked me to reach out to you."

Earlier in the email chain, a note allegedly from Paul himself read:

Alexandra,

Please do me a quick favor and contact Mark English.

I've emailed Mark multiple times this past week about my TV and Radio Ad Blitz in Iowa and still haven't heard back yet.

English found the whole thing "a little dishonest."

"It kind of rubs me the wrong way that a campaign that is trying to be real is not being real at all," he said. The Paul campaign did not respond to a request for comment.

First wave of attack

Occasional misfires aside, campaigns still raise far more money off of email than any other digital platform, including social media.

That's despite the fact that social media dominate more and more of our online habits, and that more and more people communicate via app and text rather than old-fashioned email.

Fallsgraff argued it's all about the person's mindset. "When you go to check your email, you're already prepared to make those action-based decisions, in a way that isn't true when you're scrolling through your [Facebook] timeline. Something you've received might cause you to pull out your credit card," he said.

Jordan Cohen, the chief marketing officer at the online marketing company Fluent, argued for another email advantage: "Ultimately you're able to have a one-to-one conversation with an individual, rather than blasting a conversation to the masses," he said.

"It's literally a fraction of what a marketer would spend on things like television, radio, print," Cohen added.

It's also faster to shoot a fundraising note out to a large list than it is to create and air new ads.

That means emails are often campaigns' first wave of attack when a major news story happens, or when an opposing candidate says or does something that gets into the headlines.

Avoiding email burnout

But, just like all requests for money or favors, these messages can get annoying.

Fallsgraff said the Obama campaign was mindful of never wanting to harass supporters to the point that they would unsubscribe. "The subscribers on your email list are not nameless, faceless ATMs. They are real human beings who — they just care about this campaign. They care about the election. They want to be part of it."

And the Obama campaign's approach clearly worked: In addition to raising all that money, it kept tens of millions of people on its email list.

That's why, no matter how much the Republican candidates disagree with Obama himself, nearly every presidential campaign this time around is trying to take a page from the president's fundraising playbook.